1. Field of the Invention
The present invention relates to a multielement sensor for two- or three-dimensionally detecting the magnitude and the direction of a physical quantity applied from the outside, and more particularly, to a multielement sensor including a great number of sensor elements disposed at peripheral locations along a closed circular line wherein if some sensor elements have a failure, the detection result of the failed sensor element is rejected thereby ensuring that the physical quantity can be detected accurately regardless of the failure.
2. Description of Related Art
In the automobile, mechanical, and related industries, there is an increasing need for a sensor capable of accurately detecting an acceleration which is encountered by an object when various types of external forces are applied to the object. In particular, there is a need for a small-sized sensor capable of detecting such an external force for each of two- or three-dimensional components.
The sensor of such a type can be used, for example, as a sensor in a mechanism for automatically controlling the position of a car so that when the car goes into a wrong position as a result of an abrupt steering operation or due to a strong transverse wind, the car is automatically recovered into a right position. The sensor can also be used to control a crane so that it is in a stable position. Furthermore, the sensor can also be used in a mechanism for detecting the change in a flow rate of fluid flowing through a pipe and controlling the flow rate by opening and closing a valve.
FIG. 7 illustrates an example of such a sensor 11 disclosed in Japanese Patent Application Laid-open No. 8-94661, in which upper electrodes A1-A5 are disposed on the upper surface of a disk-shaped piezoelectric element 10, and a ring-shaped cavity is formed around the origin O on the lower surface so that the location of the ring-shaped cavity corresponds to the location of the upper electrodes A1-A5. The portion of the piezoelectric element 10 where the ring-shaped cavity is formed serves as a thin flexible portion. Furthermore, a lower electrode B is formed on the lower surface of the piezoelectric element 10.
In this sensor 11, if the side face of the piezo-electric element 10 is fixed to a case, then the central portion located inside the ring-shaped cavity serves as a weight suspended by the flexible portion. When a force is applied on the weight, the flexible portion is deformed, and a charge is generated on the upper electrodes A1-A5 depending on the magnitude and direction of the applied force. The electrodes A1 and A2 detect only an X-direction component of the applied force, the electrodes A3 and A4 detect only a Y-direction component, and the electrode A5 detects only a Z-direction component, independently of each other. Detection of these components makes it to determine the magnitude and direction of the force applied on the sensor.
For example, when a force Fx in the X direction is applied on the sensor 11, the flexible portion 12 is deformed in such a manner as shown in the cross-sectional view of FIG. 8. As a result, the upper electrode A1 disposed in the X direction is expanded in the X direction while the upper electrode A2 is contracted in the X direction. Furthermore, in FIG. 8, the portion on the left side of the upper electrode A5 is contracted in the X direction while the portion on the right side of the upper electrode A5 is expanded in the X direction. In this situation, the piezoelectric element located under the upper electrodes is polarized, as shown in FIGS. 9(a) and 9(b), in different manners depending on the direction of the stress which occurs in particular portions of the piezoelectric element wherein the upper electrodes A1-A5 are collectively denoted by A. As a result, charges having polarities as shown in FIG. 8 appear on the upper electrodes A1, A2, and A5, respectively. Since the upper electrode A5 is a single electrode having a shape symmetric about the origin O, charges which are generated in the manner shown in FIG. 8 are canceled out, and no charge appears as a whole on the upper electrode A5. Similarly, no charge appears as a whole on the lower electrode B because it is formed as a single and common electrode. Therefore, if a difference between the charge generated on the upper electrode A1 and that on the upper electrode A2, then the X-direction component Fx of the applied force can be determined. Similarly, the Y-direction component of the force can be determined by the upper electrodes A3 and A4.
On the other hand, when a force Fz in the Z direction is applied on the sensor 11, a deformation occurs in the sensor as shown in FIG. 10, and a corresponding charge is generated on the upper electrode A5. The charge generated on the lower electrode B is canceled out, and thus it is possible to detect the force Fz on the basis of the charge generated on the upper electrode A5. In this situation, although charges are also generated on the upper electrodes A1-A4, these charges can be canceled out by means of proper electric connections among the electrodes A1-A4 and proper calculation process so that the upper electrodes A1-A4 are used to detect components in X and Y directions.
In the sensor 11 described above, if the entire flexible portion is formed of a piezoelectric material as shown in FIG. 7, charges generated in a deformed portion of the flexible portion are scattered into other portions in which there is only a slight or absolutely no deformation. This causes a reduction in the sensitivity of the sensor.
Although it is possible to increase the sensitivity of the sensor by increasing the size of the weight so as to make the flexible plate deformable to a greater extent, this technique is unsuitable because such an increase in weight will result in an undesirable increase in the total size of the sensor, and thus it will become impossible to realize a sensor with a desired small size.
Furthermore, in the sensor 11 described above, force components in the respective directions are determined by calculation on the basis of the charges detected by the pair of electrodes A1 and A2 and the pair of electrodes A3 and A4. If some failure occurs, as in the case where a lead wire is disconnected from a certain electrode or as in the case where the flexible member made of a piezoelectric material is broken, then the calculation of the force is performed based on the incorrect detection values, and thus the result is incorrect.